I am not an expert on that, but to my knowledge it's not that you have to
have a special German font on your computer.
The problem seems to be that some computers (I guess UNIX machines) on the
way "kill" the Umlauts. Therefore some people resort to writing "a, "o, "u
instead.
Spelling of German names became less "flexible" just when the Standesamt was
introduced, mostly in 1876. A 1904 born-man had told me once that his father
and his uncle spelt the surname differently, and without knowing that he
misspelt his surname (writing it the same way like his uncle) when he was
living with his uncle. Of course, the cousins of his father used further
variants.
Maybe one of the German computer genealogists might have the correct answer
to the problem why correctly entered Umlauts change in emails?
Sincerely,
Michael Rauck